User interface devices have been widespread, which enable a user's input motion or operation by manipulating a display screen with the user's finger, stylus, etc. Notably, in recent years, an increasing number of mobile information devices typified by such as smart phones, tablet computers, electronic books or PDAs (Personal Digital Assistants) have incorporated touch panels serving as a user interface device that receives the user's finger motion as an input signal.
Irrespective of whether a touch panel is incorporated or not, a mobile information device needs to be downsized for a greater mobility. Due to this, in case a mobile information device is equipped with a user interface device in the form of a plurality of physical input keys or buttons, these keys themselves need to be reduced in size, resulting in a risk that the operability of the mobile information device becomes poorer. In contrast, in case a mobile information device is equipped with a user interface device in the form of a touch panel using a plurality of virtual buttons instead of physical buttons, these virtual buttons can be well adjusted in size depending on functions assigned to these buttons, with a greater capability of improving the operability or maneuverability of the mobile information device.
As for a user's input motion through finger touches on a touch panel, one of key factors to improve the operability of the touch panel lies in how the user is required to manipulate the touch panel for moving an integral image displayed on and within a display. Notably, in the case of a mobile information device, a display screen is limited in size, and so there are many case where information or content such as documents or pictures cannot be entirely displayed within the display screen. As a result, the user's action required for moving an image-viewable region or a display window relative to the location of full information or content, to thereby move into view a portion of the full information which has been located out of view (or has been hidden from view or unsighted) is crucial to the user.
Scrolling is known as a manner taken when full information cannot be entirely displayed on a display screen, for moving into view a portion of the full information which has been located out of view. The scrolling is defined as a user's motion of moving an integral image being displayed on and within the display screen, upward, downward, rightward or leftward, as viewed by the user, to thereby move into view a portion of the full information which has been hidden (or assumed to be hidden) from view. In general, in the case of a personal computer, an integral image can be scrolled by a user's action of moving a scroll bar disposed on the periphery of a display screen, upward, downward, rightward or leftward, rolling a wheel of a mouth, or moving the mouth while pressing the wheel.
In addition, in the case of an automotive navigation system, so-called one-touch auto-scrolling technology has been used to automatically scroll a map such that a location on the map which is beneath a user's finger touch point on a display screen, moves to the center point of the display screen. For this one-touch auto-scrolling technology, a technique has been proposed of zooming out an image on the screen while being scrolled in the above-described manner, when a time distance between adjacent touches is equal to or smaller than a threshold (see Patent Document No. 1). This technique enables locations on the map which have been located far away from an image being displayed on screen, to be scrolled to the center point of the display screen. Additionally, a technique is also known for reducing a user's chances of performing incorrect input actions when the user performs a scroll action after a stationary touch on a touch panel, and when the user performs an instant touch on the touch panel for a very short length of time (see Patent Document No. 2).
On the other hand, many mobile information devices allow a user to scroll an integral image as a result of a so-called drag motion in which the user moves a finger while holding the finger in contact with a touch panel. In addition, a technique has been also proposed of detecting a tilt angle of a mobile information device, and then, for a period during which the user continues a particular form of a touch action, scrolling a displayed image at a varying scroll rate depending on the amount of the detected tilt angle (see Patent Document No. 3).